Another user is running this copy of Opera error

Clean reinstalled Win 8.1 and, among many others, Opera 12.17 x32, copying over the old settings files. Since then, I'm often getting the "Another user is running this copy of Opera" error box in different situations, although usually not straight off in a session. The thing is that when it's a site link out of an email, for example, the site opens along with the error box. This is a single user PC (individual/admin account) and, as I recall, the Opera recommendation was always to install for multiple users, unless individual profiles needed; when I haven't, some years ago, problems ensued. So what might be going on? Thanks,

I didn't write that very clearly. Basically, with the new install I'm getting the "Another user is running this copy of Opera" even though there's only one user - and the linked pages are opening anyway. So the question is why does Opera think there are other users that require an error message, since it's set up for multiple users?

Maybe I wasn't clear enough, because of course it's running, that's the point. With Opera already running, if I click on an email link, I'm apt to get the error message (not all the time, and typically not early in a session). I don't know if it's related or even matters, but I've always avoided setting Opera 12 or earlier as the default browser by check box or direct choice in Windows CP (Set your default programs), but rather done it indirectly: CP > Default Programs > Set Program Access and Current Defaults > Custom > Use the current browser. It's always worked and avoided some other problems that arise from time to time after setting it directly. This error box is a more recent problem, say in the past year or two. I thought it might be something corrupted in Windows or Opera, and it was worse before this week's Win 8.1 reinstall, actually blocking sites from loading (which it's not doing now). But there's apparently something more basic that's allowed it to carry on, even in this partial manner.

I get that message pop up occasionally, but I've never identified exactly why.
I suspect that instead of opening a new tab or a new window in the running Opera, the link is trying to launch a new instance of Opera.
When Opera is running, try running it again, just from its normal shortcut.
If I do that it just opens a new tab in the existing window, or a new window if I have it set to do that, which is what I'd expect it to do.

Check you "Programmes and Applications" settings in your browser's preferences: as long as you mentioned email links, check that stuff - what is set to "open mail" or what it's called. My idea is there might possibly be a sort of loop in that realm.

joshl, I'm always willing to look at a new idea, but I'm not clear what yours is. What "programs and applications" in browser preferences are you referring to? Prefs > Advanced has a browser set and a programs set. While Opera is not checked as the default browser, it is the one that shows for viewing.

The settings in Preference Settings > Advanced > Programs in Opera determine which external programs are opened BY Opera, such as opening an external e-mail program if you click on an e-mail address on a web page.
Your problem is the other way around, that an external program is calling Opera incorrectly.